C03-Media_Access - Mobile Communications Chapter 3 : Media...

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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.1 Mobile Communications Chapter 3 : Media Access ± Motivation ± SDMA, FDMA, TDMA ± Aloha ± Reservation schemes ± Collision avoidance, MACA ± Polling ± CDMA ± SAMA ± Comparison
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.2 Motivation Can we apply media access methods from fixed networks? Example CSMA/CD ± C arrier S ense M ultiple A ccess with C ollision D etection ± send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks ± signal strength decreases proportional to the square of the distance ± the sender would apply CS and CD, but the collisions happen at the receiver ± it might be the case that a sender cannot “hear” the collision, i.e., CD does not work ± furthermore, CS might not work if, e.g., a terminal is “hidden”
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.3 Motivation - hidden and exposed terminals Hidden terminals ± A sends to B, C cannot receive A ± C wants to send to B, C senses a “free” medium (CS fails) ± collision at B, A cannot receive the collision (CD fails) ± A is “hidden” for C Exposed terminals ± B sends to A, C wants to send to another terminal (not A or B) ± C has to wait, CS signals a medium in use ± but A is outside the radio range of C, therefore waiting is not necessary ± C is “exposed” to B B AC
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.4 Motivation - near and far terminals Terminals A and B send, C receives ± signal strength decreases proportional to the square of the distance ± the signal of terminal B therefore drowns out A’s signal ± C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! AB C
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.5 Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) ± segment space into sectors, use directed antennas ± cell structure FDMA (Frequency Division Multiple Access) ± assign a certain frequency to a transmission channel between a sender and a receiver ± permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) ± assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time The multiplexing schemes presented in chapter 2 are now used to control medium access!
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.6 FDD/FDMA - general scheme, example GSM f 960 MHz 124 t 1 124 1 200 kHz 935.2 MHz 20 MHz 915 MHz 890.2 MHz
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.7 TDD/TDMA - general scheme, example DECT 1 2 3 11 12 1 2 3 11 12 417 μs t downlink uplink
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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 3.8 Aloha/slotted aloha Mechanism ±
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This note was uploaded on 02/28/2008 for the course CSCI 6360 taught by Professor Wu during the Spring '08 term at University of Texas at Dallas, Richardson.

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C03-Media_Access - Mobile Communications Chapter 3 : Media...

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